When placing a surgical implant in the tissue of a patient, holes must be drilled at precise locations and depths in the tissue to ensure proper placement of the implant. The placement of surgical implants, e.g., shaped as two prong staples, four prong staples, as well as multi-holed plates with screw fixation, requires that holes be predrilled in tissue at precise locations. Pre-drilling of holes can be centered on the staple prongs or plate holes, or can be offset to provide some amount of pre-loading of the implant. For example, holes may be predrilled slightly further apart than the implants prongs to provide compression across a fracture site. Conversely, holes can be predrilled closer together than the implant prongs to create distraction loading. Hole depth must also be controlled. Holes can be planned to pierce one cortex of a bone, or provide implant fixation bi-cortically.
A class of surgical implant is made from a shape memory material such as a nickel alloy known as nitinol. Material such as nitinol or exhibiting similar characteristics as nitinol can be formed into implants, which can be made to transform their shape at some specific transformation temperature. Shape memory implants can have transformation temperatures either below body temperature or slightly above body temperature.
When the transformation temperature is designed to be below that of body temperature, the implant must be held in the implantable shape during storage. Prior to use, the implant is cooled below its transformation temperature. Once it is implanted, body temperature warms the implant above its transformation temperature. Typically, a block of plastic with precise holes is included with the implant to hold it in its implantable form.
For implants where the transformation temperature of the shape memory material is designed to be at a temperature slightly above body temperature, the implant must also be shipped and stored in a plastic holder or block. Again, holes in the plastic holder keep the implant from transforming into its higher temperature shape.
Because of the different sizes and shapes of implants available, an adjustable drill guide is used to guide the drilling. However, because two and four pronged implants are available, at least two different drill guides are necessary. Furthermore, because the prongs or legs of the implants are of different length, drill bits of specific lengths must be provided to drill the holes to a specified depth.